1. Field of Invention
The present invention relates to a power supply apparatus, and more particularly, to a power supply apparatus that performs a dynamic pulse-skipping mode (PSM).
2. Description of Related Art
The design of a conventional power supply apparatus is generally based on the specification provided by a manufacturer of load units, so the output power is a preset value, and the output power must be large enough to allow the load to work normally in any case, i.e., the output power is the maximum value of the power consumed by the load. Considering the actual application, the load, such as a processor, a random access memory, and a display, is not fully loaded at any time, especially in the case of a mobile communication product that is mostly in a standby mode to save power, so the efficiency for the power supply apparatus in the whole load range is a key point in practical application. Accordingly, in recent years, a variety of power supply apparatuses are proposed in some documents. For example, in Journal of Solid-State Circuit, Vol. 39, No. 1, Page 3-14, January 2004, in the Institute of Electrical and Electronic Engineers (IEEE), a power supply apparatus is provided, wherein the output state of the power supply apparatus is sensed through a current mode, and thereby the output is adjusted accordingly.
The conventional voltage-regulating method includes: pulse-width modulation (PWM) and pulse-frequency modulation (PFM). FIG. 1 shows the relationship between the conversion efficiency and the load currents for the PWM and PFM. Referring to FIG. 1, as for power supply with light load, the power supply apparatus is designed to regulate voltage through PFM; on the contrary, as for power supply with heavy load, the power supply apparatus is designed to regulate voltage through PWM. FIG. 2 is a circuit diagram of a detector for determining the PWM or PFM in the conventional power supply apparatus. Referring to FIG. 2, according to a comparative result output by a comparator 215, a switch 230 selects to transmit a pulse-width modulation signal output by a pulse-width modulator 205 or a pulse-frequency modulation signal output by a pulse-frequency modulator 210 to drivers 220 and 225. Power transistors 235 and 240 are respectively driven by the drivers 220 and 225 to be alternately turned on. Therefore, a load current Iout flows through an inductor 245 to be supplied to a load 270. Generally, all outputs of the power supply apparatus are connected to a load capacitor 250.
Since the voltage-regulating methods of the pulse-width modulation and the pulse-frequency modulation have their own suitable load ranges, the perfect conversion efficiency cannot always be achieved within the whole load range if a single method is employed. Therefore, in the conventional art, the pulse-width modulation and the pulse-frequency modulation are integrated into a single power supply apparatus, which is switched to one of the voltage regulating methods automatically according to the load condition. However, as shown in FIG. 1, there are also low efficiency points at the handshaking section of the curves PFM and PWM.
In order to detect the current of the load 270, a sensitive resistor is connected in series between the power transistor 235 and a voltage source Vin in the conventional art. In order to reduce the power consumption, the switching technology (shown in FIG. 2) for the conventional pulse-width modulation and pulse-frequency modulation is to connect a sensitive resistor 255 and a power transistor 260 between a node 280 and the voltage source Vin in series. The current load current Iout is obtained by using the sensitive resistor 255, and the voltage of a node 285 is compared with a predetermined reference voltage Vmode, thereby achieving a simple load-sensing architecture. However, the conventional art has two serious disadvantages: (1) power consumption of the serial-connected resistor 255; (2) such architecture is only capable of switching between the pulse-width modulation and the pulse-frequency modulation by using a fixed point, and cannot detect them dynamically.